36 EDWARD MCCRADY, JR. 



Stages 5 and 6 



The third cleavage. At the end of the 4-celled stage 

 (stage 4') the total mass of the blastomeres has been reduced 

 by the elimination of deutoplasm to such an extent that they 

 no longer fill the space within the zona pellucida, but are sepa- 

 rated from one another and to some extent from the zona by 

 yolk granules (fig. 12, B). After this there is little further 

 yolk extrusion. 



Fig. 12 Photographs from Hartman ('19). A, early 4-celled stage showing 

 large blastomeres in contact with zona. B, late 4-celled stage showing blastomeres 

 much reduced by yolk extrusion. C, 14-celled protoblastocyst with yolk showing 

 liquefaction. D, 32-celled complete unilaminar blastocyst. 



The larger cells at the A-pole of the egg continue to divide 

 precociously. A 5-celled stage is formed by the division of 

 Aj. This is followed quickly by a 6-celled stage after division 

 of A 2 . In stage 5 the two larger cells are B, and B 2 still un- 

 divided. The larger cells, therefore, are now at the B-pole 

 instead of the A-pole of the egg. In the 8-celled stage 

 (stage 6) the A-pole again contains the larger cells (on ac- 

 count of the division of B x and B 2 ), though the difference is 

 less dicided than it was originally. 



